Duchamp started it with his Readymades, using existing, commercially manufactured objects and minimally assembling them to create new objects; or simply hanging them on the wall and calling them art - as if to say that naming them "art" makes them so.
I think art is dumpster diving on a more profound, less literal level, too, in the sense of making something from nothing. Seeing gold in trash. Digging ideas out of the unlikeliest corners.
Not that this is the province only of visual artists.
It's more the nature of creativity in general, be it technological invention, scientific hypothesis, or fashion innovation. In my mind, "dumpster diving" means seeing something in a new way, beyond the obvious.